What is the ability of a population to adapt. Disturbances in the equilibrium state of populations: mutations, natural selection, migrations, isolation

Among the factors of the genetic dynamics of a population that violate its equilibrium state are: the mutation process, selection, genetic drift, migration, isolation.

Mutations and natural selection

In each generation, the gene pool of the population is replenished with newly emerging mutations. Among them, there can be both completely new changes, and mutations already existing in the population. This process is called mutational pressure. The magnitude of the mutation pressure depends on the degree of mutability of individual genes, on the ratio of direct and reverse mutations, on the efficiency of the repair system, and on the presence of mutagenic factors in the environment. In addition, the magnitude of the mutational pressure is affected by the extent to which the mutation affects the viability and fertility of the individual.

Studies show that natural populations are saturated with mutant genes, which are mostly in the heterozygous state. The mutation process creates the primary genetic variability of the population, with which further action must be taken. natural selection. In the event of a change in external conditions and a change in the direction of selection, the reserve of mutations allows the population to quickly adapt to the new situation.

The selection efficiency depends on whether the mutant trait is dominant or recessive. Purification of the population from individuals with a harmful dominant mutation can be achieved in one generation if its carrier does not leave behind offspring. At the same time, harmful recessive mutations escape the action of selection if they are in a heterozygous state, and especially in cases where selection acts in favor of heterozygotes. The latter often have a selective advantage over homozygous genotypes due to a wider reaction rate, which increases the adaptive potential of their owners. With the preservation and reproduction of heterozygotes, the probability of separating recessive homozygotes simultaneously increases. Selection in favor of heterozygotes is called balancing.

A striking example of this form of selection is the situation with the inheritance of sickle cell anemia. This disease is widespread in parts of Africa. It is caused by a mutation in the gene encoding the synthesis of the b-chain of hemoglobin, in which one amino acid (valine) is replaced by another (glutamine). Homozygotes for this mutation suffer from severe anemia, almost always leading to death at an early age. The erythrocytes of such people are sickle-shaped. Heterozygosity for this mutation does not lead to anemia. Erythrocytes in heterozygotes have a normal shape, but contain 60% normal and 40% altered hemoglobin. This suggests that both alleles function in heterozygotes - normal and mutant. Since homozygotes for the mutant allele are completely eliminated from reproduction, one would expect a decrease in the frequency of the harmful gene in the population. However, in some African tribes, the proportion of heterozygotes for this gene is 30-40%. The reason for this situation is that people with a heterozygous genotype are less likely to be infected with dengue fever, which causes high mortality in these areas, compared to the norm. In this regard, selection preserves both genotypes: normal (dominant homozygous) and heterozygous. The reproduction from generation to generation of two different genotypic classes of individuals in a population is referred to as balanced polymorphism. It has an adaptive value.

There are other forms of natural selection. Stabilizing selection preserves the norm as the variant of the genotype that best meets the prevailing conditions, eliminating the deviations from it that arise. This form of selection usually operates when the population is under relatively stable conditions of existence for a long time. In contrast, motive selection retains a new trait if the resulting mutation is beneficial and gives its bearers some advantage. Selection disruptive(tearing) acts simultaneously in two directions, preserving the extreme variants of the development of the trait. Ch. Darwin gave a typical example of this form of selection. It concerns the preservation of two forms of insects on the islands: winged and wingless, which live on different sides of the island - leeward and windless.

The main result of the activity of natural selection is reduced to an increase in the number of individuals with traits, in the direction of which selection proceeds. At the same time, signs linked to them and signs that are in a correlative relationship with the first are also selected. For genes that control traits that are not affected by selection, a population can be in a state of equilibrium for a long time, and the distribution of genotypes for them will be close to the Hardy-Weinberg formula.

Natural selection operates widely and simultaneously affects many aspects of the life of the organism. It is aimed at preserving the traits that are beneficial for the organism, which increase its adaptability and give it an advantage over other organisms. In contrast, the effect of artificial selection, which takes place in populations of cultivated plants and domestic animals, is narrower and most often affects traits that are beneficial to humans, and not to their carriers.

genetic drift

Random causes have a great influence on the genotypic structure of populations. These include: fluctuations in population size, the age and sex composition of populations, the quality and quantity of food resources, the presence or absence of competition, the random nature of the sample that gives rise to the next generation, etc. Change in gene frequencies in a population for random reasons American geneticist S. Wright named genetic drift, and N.P. Dubinin is a genetic-automatic process. Sharp fluctuations in population size have a particularly noticeable effect on the genetic structure of populations - population waves, or waves of life. It has been established that in small populations, dynamic processes proceed much more intensively, and the role of chance in the accumulation of individual genotypes increases. When a population is reduced, some mutant genes can be accidentally preserved in it, while others can also be randomly eliminated. With a subsequent increase in the population size, the number of these preserved genes can increase rapidly. The drift rate is inversely proportional to the size of the population. At the time of the decrease in numbers, the drift is especially intense. With a very sharp reduction in the population, there may be a threat of its extinction. This is the so-called “bottleneck” situation. If the population manages to survive, then as a result of genetic drift, their frequencies will change, which will affect the structure of the new generation.

Genetic-automatic processes are especially clear in isolates, when a group of individuals is isolated from a large population and forms a new settlement. There are many such examples in the genetics of human populations. So, in the state of Pennsylvania (USA) lives a sect of Mennonites, numbering several thousand people. Marriages here are allowed only between members of the sect. The isolate began with three married couples who settled in America at the end of the 18th century. This group of people is characterized by an unusually high concentration of the pleiotropic gene, which in the homozygous state causes a special form of dwarfism with polydactyly. About 13% of the members of this sect are heterozygous for this rare mutation. It is likely that the “ancestor effect” took place here: by chance one of the founders of the sect was heterozygous for this gene, and closely related marriages contributed to the spread of this anomaly. In other groups of Mennonites scattered throughout the United States, such a disease has not been found.

Migrations

Another reason for changing the frequencies of genes in a population is migration. During the movement of groups of individuals and their crossing with members of another population, genes are transferred from one population to another. The effect of migration depends on the size of the group of migrants and the difference in gene frequencies between exchanging populations. If the initial frequencies of genes in populations are very different, then a significant shift in frequencies can occur. As the migration progresses, the genetic differences between populations level off. The end result of the pressure of migrations is the establishment of a certain average concentration for each mutation throughout the system of populations between which there is an exchange of individuals.

An example of the role of migrations is the distribution of genes that determine human blood groups of the system AB0. Europe is characterized by the predominance of the group BUT, for Asia - groups AT. The reason for the differences, according to geneticists, lies in the large migrations of the population that took place from East to West in the period from 500 to 1500 years. ad.

Insulation

If individuals of one population do not fully or partially interbreed with individuals of other populations, such a population experiences a process isolation. If separation is observed over a number of generations, and selection acts in different directions in different populations, then a process of differentiation of populations occurs. The process of isolation operates both at the intrapopulation and at the interpopulation level.

There are two main types of insulation: spatial, or mechanical, isolation and biological insulation. The first type of isolation occurs either under the influence of natural geographical factors (mountain building; the emergence of rivers, lakes and other water bodies; volcanic eruption, etc.), or as a result of human activity (plowing land, draining swamps, forest plantations, etc.). One of the consequences of spatial isolation is the formation of a discontinuous range of the species, which is characteristic, in particular, of the blue magpie, sable, common frog, sedge, and common loach.

biological isolation subdivided into morpho-physiological, ecological, ethological and genetic. All these types of isolation are characterized by the appearance of reproductive barriers that limit or exclude free interbreeding.

Morpho-physiological isolation occurs mainly at the level of reproductive processes. In animals, it is often associated with differences in the structure of copulatory organs, which is especially true for insects and some rodents. In plants, such features as the size of the pollen grain, the length of the pollen tube, and the coincidence of the maturation periods of pollen and stigmas play an important role.

At ethological isolation in animals, differences in the behavior of individuals during the reproductive period serve as an obstacle, for example, unsuccessful courtship of a male for a female is observed.

Environmental isolation can manifest itself in different forms: in the preference for a certain reproductive territory, in different periods of maturation of germ cells, reproduction rates, etc. For example, in marine fish migrating to breed in rivers, a special population is formed in each river. Representatives of these populations may differ in size, color, time of onset of puberty, and other features related to the reproductive process.

genetic isolation includes different mechanisms. Most often, it occurs due to violations of the normal course of meiosis and the formation of non-viable gametes. The causes of violations can be polyploidy, chromosomal rearrangements, nuclear-plasma incompatibility. Each of these phenomena can lead to limited panmixia and infertility of hybrids, and, consequently, to limiting the process of free combination of genes.

Isolation is rarely created by any one mechanism. Usually several different forms of isolation take place at the same time. They can act both at the stage preceding fertilization and after it. In the latter case, the insulation system is less economical, since a significant amount of energy resources is wasted, for example, on the production of sterile offspring.

The listed factors of the genetic dynamics of populations can act singly and jointly. In the latter case, either a cumulative effect can be observed (for example, a mutation process + selection), or the action of one factor can reduce the effectiveness of another (for example, the appearance of migrants can reduce the effect of gene drift).

The study of dynamic processes in populations allowed S.S. Chetverikov (1928) to formulate the idea genetic homeostasis. By genetic homeostasis, he understood the equilibrium state of a population, its ability to maintain its genotypic structure in response to the action of environmental factors. The main mechanism for maintaining an equilibrium state is free crossing of individuals, under the very conditions of which, according to Chetverikov, an apparatus for stabilizing the numerical ratios of alleles is laid.

The genetic processes that we have considered, occurring at the population level, create the basis for the evolution of larger systematic groups: species, genera, families, i.e. for macroevolution. The mechanisms of micro- and macroevolution are similar in many respects, only the scale of the changes taking place is different.

In nature, each existing species is a complex complex or even a system of intraspecific groups that include individuals with specific structural, physiological and behavioral features. Such an intraspecific association of individuals is population.

The word "population" comes from the Latin "populus" - people, population. Consequently, population- a set of individuals of the same species living in a certain territory, i.e. those that only interbreed with each other. The term "population" is currently used in the narrow sense of the word when talking about a specific intraspecific grouping inhabiting a certain biogeocenosis, and in a broad, general sense - to refer to isolated groups of a species, regardless of what territory it occupies and what genetic information it carries.

Members of the same population affect each other no less than the physical factors of the environment or other species of organisms living together. In populations, to one degree or another, all forms of relationships characteristic of interspecific relations are manifested, but the most pronounced mutualistic(mutually beneficial) and competitive. Populations can be monolithic or consist of subpopulation level groupings - families, clans, herds, flocks etc. Combining organisms of the same species into a population creates qualitatively new properties. Compared to the lifetime of an individual organism, a population can exist for a very long time.

At the same time, a population is similar to an organism as a biosystem, since it has a certain structure, integrity, a genetic program for self-reproduction, and the ability to autoregulate and adapt. The interaction of people with species of organisms that are in the environment, in the natural environment or under the economic control of man, is usually mediated through populations. It is important that many patterns of population ecology also apply to human populations.

population is the genetic unit of a species, the changes of which are carried out by the evolution of the species. As a group of individuals of the same species living together, the population acts as the first supraorganismal biological macrosystem. The adaptive capacity of a population is much higher than that of its constituent individuals. A population as a biological unit has certain structure and functions.

Population structure characterized by its constituent individuals and their distribution in space.

Population functions similar to the functions of other biological systems. They are characterized by growth, development, the ability to maintain existence in constantly changing conditions, i.e. populations have specific genetic and ecological characteristics.

Populations have laws that allow the limited resources of the environment to be used in this way to ensure that offspring are left. Populations of many species have properties that allow them to regulate their numbers. Maintaining optimal population under given conditions is called population homeostasis.

Thus, populations, as group associations, have a number of specific properties that are not inherent in each individual. The main characteristics of populations: number, density, birth rate, mortality, growth rate.

Populations are characterized by a certain organization. The distribution of individuals over the territory, the ratio of groups by sex, age, morphological, physiological, behavioral and genetic characteristics reflect population structure. It is formed, on the one hand, on the basis of the general biological properties of the species, and on the other hand, under the influence of abiotic environmental factors and populations of other species. The structure of populations, therefore, has an adaptive character.

The adaptive possibilities of a species as a whole as a system of populations are much broader than the adaptive features of each particular individual.

Population structure of the species

The space or area occupied by a population may be different both for different species and within the same species. The range of a population is largely determined by the mobility of individuals or the radius of individual activity. If the radius of individual activity is small, the size of the population range is usually also small. Depending on the size of the territory occupied, it is possible to distinguish three types of populations: elementary, ecological and geographical (Fig. 1).

Rice. 1. Spatial subdivision of populations: 1, range of the species; 2-4 - respectively geographical, ecological and elementary populations

There are sex, age, genetic, spatial and ecological structure of populations.

The sexual structure of the population represents the ratio of individuals of different sexes in it.

Age structure of the population- the ratio in the composition of the population of individuals of different ages, representing one or different offspring of one or several generations.

Genetic structure of the population is determined by the variability and diversity of genotypes, the frequency of variations of individual genes - alleles, as well as the division of the population into groups of genetically close individuals, between which, when crossing, there is a constant exchange of alleles.

The spatial structure of the population - the nature of the placement and distribution of individual members of the population and their groups in the area. The spatial structure of populations differs markedly between sedentary and nomadic or migratory animals.

Ecological structure of the population is the division of any population into groups of individuals interacting differently with environmental factors.

Each species, occupying a certain territory ( range) is represented on it by a system of populations. The more complex the territory occupied by a species is dissected, the more opportunities there are for the isolation of individual populations. However, to a lesser extent, the population structure of a species is determined by its biological characteristics, such as the mobility of its constituent individuals, the degree of their attachment to the territory, and the ability to overcome natural barriers.

Isolation of populations

If the members of a species constantly mix and mingle over vast areas, such a species is characterized by a small number of large populations. With poorly developed abilities for movement, many small populations are formed in the composition of the species, reflecting the mosaic nature of the landscape. In plants and sedentary animals, the number of populations is directly dependent on the degree of heterogeneity of the environment.

The degree of isolation of neighboring populations of the species is different. In some cases, they are sharply separated by uninhabitable territory and clearly localized in space, such as populations of perch and tench in isolated lakes.

The opposite variant is the continuous colonization of large territories by the species. Within the same species, there can be populations with both well-defined and blurred boundaries, and within a species, populations can be represented by groups of different sizes.

Relationships between populations support the species as a whole. Too long and complete isolation of populations can lead to the formation of new species.

Differences between individual populations are expressed to varying degrees. They can affect not only their group characteristics, but also the qualitative features of the physiology, morphology and behavior of individual individuals. These differences are created mainly under the influence of natural selection, which adapts each population to the specific conditions of its existence.

Classification and structure of populations

An obligatory sign of a population is its ability to exist independently in a given territory for an indefinitely long time due to reproduction, and not the influx of individuals from outside. Temporary settlements of different scales do not belong to the category of populations, but are considered intrapopulation subdivisions. From these positions, the species is represented not by a hierarchical subordination, but by a spatial system of neighboring populations of different scales and with varying degrees of connections and isolation between them.

Populations can be classified according to their spatial and age structure, density, kinetics, habitat persistence or change, and other ecological criteria.

The territorial boundaries of populations of different species do not coincide. The diversity of natural populations is also expressed in the variety of types of their internal structure.

The main indicators of the structure of populations are the number, distribution of organisms in space, and the ratio of individuals of different quality.

The individual traits of each organism depend on the characteristics of its hereditary program (genotype) and on how this program is realized in the course of ontogenesis. Each individual has a certain size, sex, distinctive features of morphology, behavioral features, its own limits of endurance and adaptability to environmental changes. The distribution of these traits in a population also characterizes its structure.

The structure of the population is not stable. The growth and development of organisms, the birth of new ones, death from various causes, changes in environmental conditions, an increase or decrease in the number of enemies - all this leads to a change in various relationships within the population. The direction of its further changes largely depends on the structure of the population in a given period of time.

Sexual structure of populations

The genetic mechanism of sex determination provides for the splitting of offspring by sex in a ratio of 1: 1, the so-called sex ratio. But it does not follow from this that the same ratio is characteristic of the population as a whole. Sex-linked traits often determine significant differences in the physiology, ecology, and behavior of females and males. Due to the different viability of the male and female organisms, this primary ratio often differs from the secondary and especially from the tertiary ratio, which is characteristic of adults. So, in humans, the secondary sex ratio is 100 girls to 106 boys, by the age of 16-18 this ratio is leveled off due to increased male mortality and by the age of 50 it is 85 men per 100 women, and by the age of 80 - 50 men per 100 women.

The sex ratio in a population is established not only according to genetic laws, but also to a certain extent under the influence of the environment.

Age structure of populations

Birth and death rates, population dynamics are directly related to the age structure of the population. The population consists of individuals of different age and sex. For each species, and sometimes for each population within a species, its own ratios of age groups are characteristic. In relation to the population, they usually distinguish three ecological ages: pre-reproductive, reproductive and post-reproductive.

With age, the requirements of an individual to the environment and resistance to its individual factors naturally and very significantly change. At different stages of ontogenesis, a change in habitats, a change in the type of nutrition, the nature of movement, and the general activity of organisms can occur.

Age differences in the population significantly increase its ecological heterogeneity and, consequently, its resistance to the environment. The probability increases that in case of strong deviations of conditions from the norm, at least a part of viable individuals will remain in the population, and it will be able to continue its existence.

The age structure of populations has an adaptive character. It is formed on the basis of the biological properties of the species, but always also reflects the strength of the impact of environmental factors.

Age structure of populations in plants

In plants, the age structure of the cenopopulation, i.e. population of a particular phytocenosis is determined by the ratio of age groups. The absolute, or calendar, age of a plant and its age state are not identical concepts. Plants of the same age can be in different age states. The age or ontogenetic state of an individual is the stage of its ontogenesis, at which it is characterized by certain relationships with the environment.

The age structure of the cenopopulation is largely determined by the biological characteristics of the species: the frequency of fruiting, the number of produced seeds and vegetative primordia, the ability of vegetative primordia to rejuvenate, the rate of transition of individuals from one age state to another, the ability to form clones, etc. The manifestation of all these biological features, in turn, turn depends on the environmental conditions. The course of ontogenesis also changes, which can occur in one species in many variants.

Different plant sizes reflect different vitality individuals within each age group. The vitality of an individual is manifested in the power of its vegetative and generative organs, which corresponds to the amount of accumulated energy, and in resistance to adverse effects, which is determined by the ability to regenerate. The vitality of each individual changes in ontogenesis along a single-peak curve, increasing on the ascending branch of ontogenesis and decreasing on the descending one.

Many meadow, forest, steppe species when grown in nurseries or crops, i.e. on the best agrotechnical background, reduce their ontogeny.

The ability to change the path of ontogenesis ensures adaptation to changing environmental conditions and expands the ecological niche of the species.

Age structure of populations in animals

Depending on the characteristics of reproduction, members of a population may belong to the same generation or to different ones. In the first case, all individuals are close in age and approximately simultaneously go through the next stages of the life cycle. The timing of reproduction and the passage of individual age stages are usually confined to a specific season of the year. The size of such populations is, as a rule, unstable: strong deviations of conditions from the optimum at any stage of the life cycle affect the entire population at once, causing significant mortality.

In species with a single reproduction and short life cycles, several generations are replaced during the year.

When human exploitation of natural populations of animals, taking into account their age structure is of paramount importance. In species with a large annual recruitment, a larger part of the population can be removed without the threat of undermining its numbers. For example, in pink salmon, which matures in the second year of life, it is possible to catch up to 50-60% of spawning individuals without the threat of further population decline. For chum salmon that matures later and has a more complex age structure, the removal rates from a mature herd should be lower.

An analysis of the age structure helps to predict the size of the population over the life of a number of next generations.

The space occupied by the population provides it with the means of subsistence. Each territory can feed only a certain number of individuals. Naturally, the completeness of the use of available resources depends not only on the total size of the population, but also on the distribution of individuals in space. This is clearly manifested in plants whose feeding area cannot be less than a certain limiting value.

In nature, an almost uniform ordered distribution of individuals in the occupied territory is occasionally found. However, most often the members of the population are distributed unevenly in space.

In each specific case, the type of distribution in the occupied space turns out to be adaptive, i.e. allows optimal use of available resources. Plants in a cenopopulation are most often distributed extremely unevenly. Often the denser center of the cluster is surrounded by less densely spaced individuals.

The spatial heterogeneity of the cenopopulation is related to the nature of the development of clusters in time.

In animals, due to their mobility, the methods of ordering territorial relations are more diverse than in plants.

In higher animals, intrapopulation distribution is regulated by a system of instincts. They are characterized by a special territorial behavior - a reaction to the location of other members of the population. However, sedentary life is fraught with the threat of rapid depletion of resources if the population density is too high. The total area occupied by the population is divided into separate individual or group areas, which achieves an orderly use of food supplies, natural shelters, breeding grounds, etc.

Despite the territorial isolation of the members of the population, communication is maintained between them using a system of various signals and direct contacts at the borders of possessions.

"Securing the site" is achieved in various ways: 1) by protecting the boundaries of the occupied space and by direct aggression towards the stranger; 2) special ritual behavior that demonstrates a threat; 3) a system of special signals and marks indicating the occupation of the territory.

The usual reaction to territorial marks - avoidance - is hereditary in animals. The biological benefit of this type of behavior is clear. If the possession of a territory was decided only by the outcome of a physical struggle, the appearance of each stronger newcomer would threaten the owner with the loss of the territory and elimination from reproduction.

Partial overlap of individual territories serves as a way to maintain contacts between members of the population. Neighboring individuals often maintain a stable mutually beneficial system of connections: mutual warning of danger, joint protection from enemies. The normal behavior of animals includes an active search for contacts with members of their own species, which often intensifies during a period of decline in numbers.

Some species form widely nomadic groups that are not tied to a specific territory. This is the behavior of many fish species during feeding migrations.

There are no absolute distinctions between different ways of using the territory. The spatial structure of the population is very dynamic. It is subject to seasonal and other adaptive rearrangements in accordance with place and time.

The patterns of animal behavior are the subject of a special science - ethology. The system of relationships between members of one population is therefore called the ethological or behavioral structure of the population.

The behavior of animals in relation to other members of the population depends, first of all, on whether a solitary or group way of life is characteristic of the species.

A solitary lifestyle, in which the individuals of a population are independent and isolated from each other, is characteristic of many species, but only at certain stages of the life cycle. Completely solitary existence of organisms does not occur in nature, since in this case it would be impossible to carry out their main vital function - reproduction.

With a family lifestyle, the bonds between parents and their offspring are also strengthened. The simplest type of such a connection is the care of one of the parents about the laid eggs: guarding the clutch, incubation, additional aeration, etc. With a family lifestyle, the territorial behavior of animals is most pronounced: various signals, markings, ritual forms of threat and direct aggression provide possession of a plot sufficient for rearing offspring.

Larger associations of animals - flocks, herds and colonies. Their formation is based on the further complication of behavioral relationships in populations.

Life in a group through the nervous and hormonal systems is reflected in the course of many physiological processes in the animal's body. In isolated individuals, the level of metabolism noticeably changes, reserve substances are used up faster, a number of instincts do not manifest themselves, and overall viability worsens.

Positive group effect manifests itself only up to a certain optimal level of population density. If there are too many animals, it threatens everyone with a lack of environmental resources. Then other mechanisms come into play, leading to a decrease in the number of individuals in the group through its division, dispersal, or a drop in the birth rate.

Goals: to form the concept of a population as an elementary unit of evolution; show the role of hereditary variability as one of the evolutionary factors, the causes of species variability.

move lesson

I. Check of knowledge.

1. Testing.

1) The presence of similar structural features of organisms determines

criterion:

a) genetic;

b) morphological;

c) physiological;

d) ecological.

2) The commonality of ancestors proves the criterion:

a) historical;

b) morphological;

c) genetic d) geographical.

3) The karyotype of organisms studies the criterion:

a) genetic:

b) physiological;

c) morphological; d) historical.

4) The influence of biotic environmental factors on organisms considers the criterion:

a) geographic; b) environmental;

c) physiological;

d) history.

5) The distribution of species in nature considers the criterion:

a) ecological;

b) geographical; c) historical;

d) physiological.

6) Differentiation of species according to the set of enzymes is carried out in accordance with:

a) with a morphological criterion;

b) physiological criterion;

c) biochemical criterion;

d) genetic criterion.

7) the ability of organisms to produce fertile offspring

serves as the basis for:

a) for morphological criterion; b) physiological criterion;

c) genetic criterion;

d) ecological criterion.

8) The similarity of the processes of nutrition and respiration studies the criterion:

a) ecological;

b) physiological;

c) biochemical;

d) genetic.

9) The totality of environmental factors is the basis:

a) genetic criterion;

b) geographical criterion;

c) ecological criterion;

d) historical criterion.

2. Written answer on the card.

Exercise.

Fill in the gaps in the following phrases:

1) The set of environmental factors in which the species exists is ... the criterion of the species

2) The main reason for isolating a group of individuals into a population is ...

3) Individuals of two populations of the same species ...

5) The similarity of the body's reactions to external influences, the rhythms of development and reproduction studies ... criterion

II. Learning new material.

1 Populations.

Living organisms in nature, as a rule, do not live alone, but form more or less permanent groups. There are many reasons for the formation of such groups, but the main ones are that organisms belonging to the same species accumulate in places most favorable for their existence and reproduction.

A set of individuals of the same species that inhabit a certain space for a long time, multiply by free crossing and to some extent isolated from each other, is called a population.

The existence of species in the form of populations is a consequence of the heterogeneity of external conditions. Populations remain stable in time and space, although their numbers may vary from year to year due to differences in the conditions of reproduction and development of organisms. Within populations, there are still smaller troupes, into which individuals with similar behavior or on the basis of family ties can unite. However, they are not able to sustainably support themselves.

The organisms that make up a population are related to each other in various ways. They compete with each other for certain types of resources. Internal relationships in populations are complex and contradictory. Within each population of sexually reproducing organisms, there is a constant exchange of genetic material.

Interbreeding of individuals from different populations occurs less frequently, so the genetic exchange between different populations is limited. As a result, each population is characterized by its own specific set of genes with a ratio of frequencies of occurrence of different alleles inherent only to this population. The existence of species in the form of populations increases their resistance to local changes in living conditions.

2. Population genetics.

In Darwin's time, genetics did not exist. It as a science began to develop in the twentieth century. it became known that the carriers of hereditary variability are genes. Representations of genetics have made in-depth explanations in the theory of natural selection by Charles Darwin. The synthesis of genetics and classical Darwinism led to the birth of population genetics, which made it possible to explain from new positions the processes of changing the genetic composition of populations, the emergence of new properties of organisms and their consolidation under the influence of natural selection.

A population is a collection of organisms of the same species, each of which has a certain genotype. The totality of the genotypes of all individuals of a population is called the gene pool of the population. The richness of the gene pool depends on allelic diversity. This means that in a population where there is no allelic diversity for a particular gene, all individuals have an identical genotype for that AA gene. Genes that have two or more allelic variants in a population are called polymorphic. With two alleles, there are three genotypes (AA, Aa, aa), with three alleles - six genotypes, and then their number increases rapidly.

The richness of the gene pool of a species is determined not only by allelic diversity, that is, by the polymorphism of loci, but also by the diversity of combinations of alleles. A sharp decrease in the number of species leads to a reduction in allelic diversity and the number of combinations. Therefore, it is important to preserve the gene pools of wild species, to prevent a sharp depletion. The intensity of the processes occurring in populations largely depends on the level of genetic diversity.

The mutation process is the source of hereditary variability. In a population consisting of several million individuals, several mutations of each gene present in this population may occur in each generation. Due to combinative variability, mutations spread in a population.

The constantly ongoing mutation process and free crossing lead to the accumulation of a large number of qualitative changes that do not appear outwardly (the vast majority of emerging mutations are recessive) within the population. These facts were established by the Russian scientist S. S. Chetverikov.

Genetic studies of natural populations of plants and animals have shown that, despite their relative phenotypic homogeneity, they are saturated with various recessive mutations. Chromosomes in which mutations occurred, as a result of doubling during cell division, gradually spread among populations. Mutations do not appear phenotypically as long as they remain heterozygous.

Upon reaching a sufficiently high concentration of mutations, it becomes possible for individuals carrying allelic recessive genes to interbreed.

In these cases, mutations manifest themselves phenotypically and fall under the direct control of natural selection, and this is precisely the ability of the population to adapt, that is, adapt to new factors - climate change, the emergence of a new predator or competitor, and even to human pollution.

III. Consolidation.

Laboratory work

Topic: DETECTION OF VARIABILITY IN INDIVIDUALS OF THE SAME SPECIES

Goals: to form the concept of the variability of organisms, to continue the development of skills to observe natural objects, to find signs of variability.

Equipment: handout illustrating the variability of organisms (plants of 5-6 species, 2-3 specimens of each species, sets of seeds, fruits, leaves, etc.)

Progress

1. Compare 2-3 plants of the same species (or their individual organs: leaves, seeds, fruits, etc.). Find signs of similarity in their structure. Explain the reasons for the similarity of individuals of the same species.

2. Identify signs of difference in the studied plants. Answer the question: what properties of organisms cause differences between individuals of the same species? 3. Expand the meaning of these properties of organisms for evolution. What, in your opinion, differences are due to hereditary variability, which are not hereditary variability? Explain how differences between individuals of the same species could have arisen.

Homework: § 54, 55.

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Biology Lesson Plan

Topic: Genetic composition of populations

genetics mutational hereditary population

Type of lesson: a lesson that reveals the content of the topic.

The purpose of the lesson: continue to deepen and expand knowledge about populations, to characterize the concept of the gene pool of populations.

Tasks:

Educational. To form the concept of population genetics; characterize the gene pool of a population; find out that the mutation process is a constant source of hereditary variability.

Developing. Continue to form the ability to observe and note the main thing when listening to messages, working with textbook material.

Educational. Continue to form a scientific outlook, love for nature, work culture based on keeping records in a notebook.

Equipment

Tables, textbook.

During the classes

1. Organizational moment 1-2 min. Homework survey: 1) What is a population? 2) Why do biological species exist in the form of populations? 5-7 min.

2. Learning new material. 25 min.

3. Consolidation of the studied material. Grading.

4. Homework.

2. Learning new material

Consolidation of the studied material

4. Homework

population genetics. At the time of Darwin, the science of genetics did not yet exist. It began to develop at the beginning of the 20th century. It became known that the carriers of hereditary variability are genes.

Representations of genetics have introduced additional in-depth explanations into the theory of natural selection by Charles Darwin. The synthesis of genetics and classical Darwinism led to the birth of a special area of research - population genetics, which made it possible to explain from new positions the processes of changing the genetic composition of populations, the emergence of new properties of organisms and their consolidation under the influence of natural selection.

Gene pool. Each population is characterized by a certain gene pool, i.e. the total amount of genetic material that is made up of the genotypes of individual individuals.

The necessary prerequisites for the evolutionary process are the occurrence of elementary changes in the apparatus of heredity - mutations, their distribution and fixation in the gene pools of populations of organisms. Directed changes in the gene pools of populations under the influence of various factors are elementary evolutionary changes.

As already noted, natural populations in different parts of the range of a species are usually more or less different. Within each population there is free interbreeding of individuals. As a result, each population is characterized by its own gene pool with ratios of various alleles inherent only to this population.

The mutation process is a constant source of hereditary variability. In a population consisting of several million individuals, several mutations of literally every gene present in this population can occur in each generation. Due to combinative variability, mutations spread in a population.

Natural populations are saturated with a wide variety of mutations. This was noticed by the Russian scientist Sergey Sergeevich Chetverikov (1880-1959), who found that a significant part of the variability of the gene pool is hidden from view, since the vast majority of the resulting mutations are recessive and do not appear outwardly. Recessive mutations seem to be “absorbed by a species in a heterozygous state”, because most organisms are heterozygous for many genes. Such latent variability can be revealed in experiments with crossing closely related individuals. With such a cross, some recessive alleles that were in a heterozygous and therefore latent state will go into a homozygous state and will be able to appear.

Significant genetic variability of natural populations is easily detected in the course of artificial selection. In artificial selection, those individuals are selected from a population in which any economically valuable traits are most pronounced, and these individuals are crossed with each other. Artificial selection is effective in almost all cases when it is resorted to. Consequently, in populations there is genetic variability for literally every trait of a given organism.

The forces that cause gene mutations operate randomly. The probability of a mutant individual appearing in an environment in which selection will favor it is no greater than in an environment in which it will almost certainly perish. S.S. Chetverikov showed that, with rare exceptions, most of the newly emerging mutations are harmful and in the homozygous state, as a rule, reduce the viability of individuals. They persist in populations only through selection in favor of heterozygotes. However, mutations that are detrimental in some conditions may increase viability in other conditions. Thus, a mutation that causes the underdevelopment or complete absence of wings in insects is certainly harmful under normal conditions, and wingless individuals are quickly replaced by normal ones. But on oceanic islands and mountain passes, where strong winds blow, such insects have advantages over individuals with normally developed wings.

Since any population is usually well adapted to its environment, major changes usually reduce this fitness, just as large accidental changes in the mechanism of a clock (removal of some spring or addition of a wheel) lead to its failure. There are large stocks of such alleles in populations that do not bring it any benefit at a given place or at a given time; they remain in the population in a heterozygous state until, as a result of a change in environmental conditions, they suddenly turn out to be useful. As soon as this happens, their frequency under the influence of selection begins to increase, and eventually they become the main genetic material. This is where the population's ability to adapt lies, i.e. adapt to new factors - climate change, the emergence of a new predator or competitor, and even human pollution.

An example of such adaptation is the evolution of insecticide-resistant insect species. Events in all cases develop in the same way: when a new insecticide (a poison that acts on insects) is introduced into practice, a small amount of it is enough to successfully control an insect pest. Over time, the concentration of the insecticide has to be increased until, finally, it is ineffective. The first report of insecticide resistance in insects appeared in 1947 and related to housefly resistance to DDT. Subsequently, resistance to one or more insecticides has been found in at least 225 species of insects and other arthropods. Genes capable of providing resistance to insecticides were apparently present in each of the populations of these species; their action and ensured the ultimate reduction in the effectiveness of poisons used for pest control.

Thus, the mutation process creates material for evolutionary transformations, forming a reserve of hereditary variability in the gene pool of each population and species as a whole. By maintaining a high degree of genetic diversity in populations, it provides the basis for the operation of natural selection and microevolution.

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Portal for the student. Self-training

What is the ability of a population to adapt. Disturbances in the equilibrium state of populations: mutations, natural selection, migrations, isolation

Mutations and natural selection

genetic drift

Migrations

Insulation

Population structure of the species

Isolation of populations

Classification and structure of populations

Sexual structure of populations

Age structure of populations

Age structure of populations in plants

Age structure of populations in animals

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Portal for the student. Self-training

Mutations and natural selection

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Classification and structure of populations

Sexual structure of populations

Age structure of populations

Age structure of populations in plants

Age structure of populations in animals

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